1. Field of the Invention
The present invention relates to a smoke detecting apparatus which determines a fire by optically detecting smoke particles suspended in air sucked from a monitored zone, by means of laser light.
2. Description of the Related Art
Conventionally, a supersensitive smoke detecting apparatus is used for detecting a fire at an extremely early stage in a clean space which is represented by a computer room or a clean room for semiconductor manufacturing equipment.
In such a supersensitive smoke detecting apparatus, air is sucked through a pipe disposed in the clean space. Smoke particles contained in the sucked air are passed through a smoke detecting area which is irradiated with laser light from a laser diode. In the light reception pulse signal due to light scattered by the smoke particles and detected by a light receiving device, pulses having a level which exceeds a predetermined threshold value are counted for a unit time period. Based on the count number per unit time period, even a very low smoke density in the range of, for example, 0.05 to 0.20%/m is detected.
In such a supersensitive smoke detecting apparatus which detects the smoke density by counting pulses of the light reception pulse signal, however, the count number of scattered light per unit time period is varied by fluctuations of the flow rate of the sucked air. This results in a problem in that the smoker density cannot be accurately detected.
In order to solve the problem, in the conventional apparatus, the flow rate of the sucked air is measured by a flowmeter, a correction coefficient is obtained from a preset flow rate and the detected flow rate, and a count number per unit time period is corrected. Specifically, when the actual detected flow rate Q is higher than the preset flow rate Qr, the count number per unit time period is increased, and thus the smoke density is detected to be rather higher. Therefore, a correction coefficient K is obtained by an expression of K=Qr/Q. The count value per unit time period is multiplied by the correction coefficient, to be corrected to the count value converted for the preset flow rate. Accordingly, the smoke density can be correctly detected.
However, the correction of the count number per unit time period to the variation in the flow rate of the sucked air necessitates a flowmeter, and the production cost of the apparatus is remarkably increased. In addition, when any failure occurs in the flowmeter, there arises another problem in that the smoke density cannot be accurately detected.